Clean up unit tests a bit.

Summary:
Splitting out some housekeeping undertaken while writing tests for D933.
* Remove miscellaneous cruft.
* Add fixtures that commit linked Message, Thread, Folder, and ImapUid objects,
  for more concise tests.

Test Plan: Run the tests.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D932